macOS High Sierra (10.13.x) introduced a new security feature that limits new third-party kernel extensions. The feature will require intervention on the users part to allow Symantec Endpoint Protection (SEP) to function fully on macOS High Sierra.
3. Open "Security" from System Preferences and then select "Allow" next to the message "System software from developer 'Symantec' was blocked from loading."
4. Save your work and restart your computer. Upon login reopen SEP from the mac title bar again and verify that SEP is functioning correctly by stating "Your Computer is protected."